Vacuum deposition as a method of creating deposits of a film on a surface is known and has been done for a number of years. The sputtering process, which is one form of deposition, involves the ionization of inert gas molecules which, under the effect of a electric field, then impact on a source material and dislodge particles of the source material. These dislodged particles have high energy levels and travel toward a substrate where they react with other atoms or molecules and are deposited on the substrate. The process is effective and relatively simple but it suffers from a variety of serious problems.
The other form of vacuum deposition is an evaporation process. The layers deposited in this prior art evaporation process were often porous and often absorbed moisture which changed their optical properties.
A solution for the porosity is to sputter the coatings which provides better density of deposit, but as mentioned above, sputtering has several serious drawbacks. The sputtering process requires the presence of an inert gas yet this gas causes thermalization of the sputtered particles due to collisions between the ions and molecules and this in turn lowers the energy levels of the sputtered particles. High energy of deposited particles gives best properties of the coatings so collisions are to be avoided. The possibility of collisions and the resulting loss of energy in the particles in this process requires the substrate and the source to be close together to avoid collisions.
Testing is also difficult when the source and the substrate are close together. At present, complex and tight tolerance coating where a 0.3% center wavelength or optical thickness variation or bandpass variation is not done due to the complexity of maintaining coating uniformity and the need to test as the coating is deposited.
One of the ways the uniformity of the coating is improved that is utilized in evaporative coating is the box coater. In this generally available system, a large square chamber is used with a circle of devices located on the bottom of the chamber. In this circle are two thermal sources and two electron guns in a typical arrangement. The multiple sources and the fact that these sources are located at a considerable distance from the substrates being coated and the fact that the substrates are rotated make the uniformity of the coating usually good. Unfortunately, the distances in a box coater are an insurmountable barrier to prior art sputtering processes since there would be many collisions between the particles sputtered from the surface of the source and the inert gas on the way to the substrate. The presence of inert gas lowers the mean free path between collisions and the many collisions destroy the energetic particles energy and remove a major advantage of the sputtering process. Ideally there should be two or less collisions between the substrate and the sputtering source for the average particle that is being coated on the substrate. In a box coater configuration, ten or more collisions would be normal due to the effect of reactant gas and inert gas.
The biggest problem with using large spacing between the substrate and source in sputtering processes is thermalization. The sputtering process is effected by energized molecules of inert gas impacting a source surface and displacing particles of the source material. This particle then travels from the area of the source toward the deposition substrate where it hits and combines with another reactant on the surface of the substrate and is deposited on the substrate. As the particle travels from the source toward the substrate, it constantly encounters other molecules of the inert gas. The inert gas must be present in sufficient concentration to effectively displace particles from the source but, after that function is complete, the inert gas concentration is destructive to the transfer of the particles to the substrate.
The basic method of reducing the thermalization and scattering is the reduction of the spacing between the substrate and the source to reduce the number of total collisions. The large spacings used in a box coating configuration, or an approach to point source geometry, is thus not possible in sputtering of coatings at present. When the space between the substrate and the source is reduced one new problem is created. The density of the stream of particles is now not uniform in relation to various points on the substrate. If the areas of the substrates are large in relation to the source area, it is to be expected that the layer of deposit depends on the path distance variations from substrate to source, Layer variation will be a function of the impingement or condensation amounts and thus vary in thickness. The suggested cure fop this positional layer thickness problem is to use relatively complex rotation of the substrates.
The closeness of the source and the substrate is the cause of another basic problem. The deposition often involves a reaction of one element from the source and another element that is introduced into the area of the substrate. The source material and a reactive gas (reactant) are the most common combination of materials that are induced to react on the substrate to form the desired final layer composition. The reactant has a drastic effect on the source if any reaction takes place on the source. This poisoning of the source has the effect of a roughly tenfold diminution of the number of particles emitted from the surface of the source.
There are several ways to decrease the poisoning of the source. One popular method is to maintain a higher partial pressure of the inert gas in the area of the source and a higher partial pressure of the reactant in the substrate area. This is difficult to do. The close spacing between source and substrate provides opportunity for increased poisoning of the source.
There are several other complex solutions that involve rotation of the substrates between physically separated zones where either reactant or source particles and inert gas predominate. An ideal system would keep the reactant and the inert gas separate and have little gas in the chamber. Since the gas must be present in at least a stoichiometric amount and since it can easily travel in the vacuum chamber, a goal of very low molecular density of gas is not easy to accomplish.
There is no good solution for the separation of reactant and source. In addition, the need for specific densities of molecules at the source and in the reaction area make very low vacuum levels impractical.
An ideal system would have practically no possibility of the reactant reaching the source to eliminate the poisoning of the source, would have very large spaces between the substrate and the source so the geometrical variation in total path is minimized, and would operate at vacuum levels that with the present technology would not allow enough impacts on the source to provide reasonable transfer of the source materials.
Another problem with prior art sputtering processes is the need to have the source and the substrate close together to reduce collision between the sputtered particles, inert gas molecules and other particles which cause energy reduction in the sputtered particles. The ways to reduce the collisions are reduction in total path and reduction in the total density of particles other than source derived particles. Path reduction requires the source be as close as possible to the substrate. Density reduction requires higher vacuum levels.
When the total path is reduced the uniformity of the coating is much more affected by geometry of the parts being coated and the source. The ideal for uniform coatings is to have the source as far away as practical since distance reduces the effect of geometry.
The use of distance as a way to reduce the geometry effects is not generally acceptable since greater distances give longer paths and more collisions which reduce the energy of the sputtered particles. Thus, there is a need for a method and apparatus that can allow a source located at a distance from the substrate and that still allows the sputtered particles to retain their energy level.
The reduction in the path length flies in the face of the need for a longer path to reduce problems in geometry. The density of the gas in the vacuum chamber is tied to two needs. One need is for enough inert gas at the source to enable the sputtering process. Another need is for enough reactive gas at the substrate to react the sputtered particles to the desired coating. The density needed for both the sputtering and the reaction provide enough molecules in the vacuum chamber to create a short mean free path between collisions for the sputtered molecules. As a result of collisions, gas is heated in the chamber and the sputtered particles are depleted of their energy rapidly due to momentum exchange. The source and the substrate must be close to each other to reduce this loss of energy. The normal distance between parts and source is thus forced into the 2 to 5 inch range for most uses.
Thus, there is a need for a method and apparatus that allows a reduced density of gas in the vacuum chamber and thus allows greater distance between the source and the substrate.
There have been attempts at reducing the reactive gas density by segregation of the reactant gas and the inert gas in chambers. One interesting method of doing this is shown in U.S. Pat. No. 4,851,095, supra, which uses a rotary drum to shuttle substrates between high reactant concentration zones and between the sputtering zone. This process creates a needless complexity and will not handle large substrates.
Thus, there is a need for a simple process that will allow a large spacing between the substrate and the source and still permit sputtered particles to reach the substrate with high energy levels. There is also a need for thickness monitoring on real time basis of uniform layers and an ability to coat large and non uniform shaped parts.
There is one specific technology that needs a large spacing between the source and the substrate. In rain erosion coatings of infrared windows and domes, the parts that need coating are frequently large and awkward curved surfaces, such as nose cones, and outer surfaces of aircraft. In these applications, the need is acute for a method to coat the large surfaces with a water and impact resistant layer that holds up under impacts of rain and has good infrared transparency. Here the shape and size make a close spacing between the source and the substrate very difficult. As a result of size and shape considerations, completely different techniques, such as use of microwave degradation of gas and pyrolysis methods, are often used to coat infrared domes.
Thus, there is also a need to be able to coat large non-flat surfaces that has not been met with magnetron sputtering methods. This can be done only when larger spacing is possible between sources and substrates in sputtering.
Two basic methods are used when a coating of a compound is desired.
In the simpler method, the compound is excited to displace molecules and the displaced molecules of the compound are then deposited on the objects. The excitation can be by gas molecules where an inert gas is excited and which then impinges on the source, by thermal methods or by several other methods.
In another method, the object of excitation is a metal as a precursor to the desired coating compound, and the actual reaction is a surface reaction between the excited and displaced precursor material and a reactant gas that is introduced into the vacuum chamber.
The second method of deposition is of interest because it uses sputtering processes. The introduction of reactant gas into a vacuum chamber created major problems. The gas is not restrained within the chamber so it can react on the surface of the source precursor material as well as on the surface of the coated object. Even a slight surface reaction on the source precursor material grossly slows the rate of reaction by blocking surface emission of the precursor.
There have been several methods to stop the poisoning of the precursor material most of which involve rotating the objects to be coated between zones, one where the precursor was deposited on the surface and the next with the reactive gas.
A typical method for optical coating as mentioned above has been the evaporation of a compound but the evaporated coatings when formed are rarely the dense and perfect coatings expected. There can be variations in the source material or geometry or flux around the source material which gives directionality to the evaporated molecules emitted from the source. The material can deposit in a porous form that is often 85% or less of the theoretical bulk density of the coating compound. The coatings are seemingly formed of columns of molecules and spaces can exist between the columns according to one theory. The surface formed with the pores attracts water vapor which can have a major effect on performance of the coating. Thereis a need to form a coating that is dense and relatively pore free.